Field of the Invention
The present invention relates to a breast tomography apparatus and a control method.
Description of the Related Art
As an apparatus for finding breast cancer, a breast-specific tomography apparatus (to be referred to as a breast tomography apparatus hereinafter) is available. In this breast tomography apparatus, the breast is inserted into the breast insert portion provided in a gantry, and a radiation generator and a radiation detector are rotated to acquire a plurality of projection images of the breast. A tomographic image is then acquired by reconstructing the projection images. In order to acquire tomographic images sufficient for diagnosis, such a breast tomography apparatus needs to acquire images of the overall breast including a portion near the chest wall of the human body. For this purpose, it is necessary to check, before the start of projection imaging, that the overall breast including a portion near the chest wall is sufficiently deeply inserted into the breast insert portion. If the breast is not sufficiently deeply inserted, it is necessary to perform re-imaging upon checking the insertion of the breast again so as to image a portion near the chest wall.
Japanese Patent Laid-Open No. 2013-22041 (to be referred to as patent literature 1 hereinafter) discloses a method of checking such an insert condition of the breast. In addition, methods of detecting body motions are disclosed in patent literature 1 and Japanese Patent Laid-Open No. 2007-82908 (to be referred to as patent literature 2 hereinafter). Patent literature 1 discloses a technique of specifying the position of a breast of an object by detecting the depth from the insert opening. This literature also describes a technique of detecting a change in the depth of the breast and monitoring the body motion of the object by always monitoring the depth. Patent literature 2 discloses a method of determining the presence/absence of a body motion by performing subtraction with respect to a given acquired image and an image acquired before the acquired image for each pixel and comparing the sum of the absolute values of the differences with a threshold.
There are individual difference in breast size among objects. For this reason, even breasts inserted sufficiently deeply differ in the depth from the insert opening. If, for example, the size of the breast is larger than the average size, the depth from the insert opening, which the breast reaches, is larger than the average depth. This makes it sometimes inaccurate to determine that the overall breast is sufficiently deeply inserted into the imaging target insert portion by using the method disclosed in patent literature 1.
According to patent literature 2, it is necessary to save an image for subtraction other than an obtained image in a storage device, among storage devices, which enables fast access, such as a DRAM (Dynamic Random Access Memory). Such a storage device enabling fast access has its own limit on storage capacity. If, therefore, this storage device holds images used for subtraction for the above body motion detection, the storage capacity used for other purposes decreases. In addition, a large amount of images are acquired in tomography, and hence performing subtraction with respect to all the pixels leads to a decrease in image processing speed.